Evaluating the surface properties of HA coating on titanium-aluminum substrate

Abstract

Hydroxyapatite (HA) is the main structural component in natural bone. Due to its excellent biocompatibility and bioactivity, it can be used for biomedical application as a coating layer for metallic implants. It is reported that it helps the formation of chemical bonding at the HA/bone interface. It also works as a protective layer against ion release from a metallic prosthesis. In this project, HA bioactive coatings were prepared via sol-gel method on Ti-Al-Nb substrate. Although sol-gel is simple to control the chemical composition and able to coat on the complex-shape implants, massive cracks of HA sol-gel coated layer on implants are still remain as a major issue. Cracks can be minimized by changing the viscosity, composition or variation in heat treatment procedure. In this research,Na3PO4 and CaCl2were used as biocompatible additives for preparing the sol-gel. The sol-gel solution was centrifuged at two different speeds (1000 and 3000 rpm) to remove excessive water contents to a desired viscosity. Substrates were then dipped into sol-gel at 3 m/s for 20 sec and dried at room temperature for 72hrs. They were sintered at 500°C, 600°C and 700°C for 15 and 45 minutes. It is found that increasing of sintering temperature and time from 500ºC to 600ºC and from 15 to 45 minutes respectively, the hardness value of sintered HA has increased significantly. However, there is a decrease in HA hardness value when it was sintered at 700°C. Surface characteristics have been analyzed by FESEM and AFM equipments. The AFM results show that surface roughness of the Ti-Al-Nb coated with HA decreases when sintering temperature is increased from 500oC to 600°C. However it is concluded that sintering temperature above 600°C for HA coating on Ti-Al-Nb results in massive cracks and worse surface roughness. Moreover, the effect of HA coated layer on corrosion behavior of metallic substrates was evaluated which results better corrosion resistance of HA coated Ti-Al-Nb substrate compare to uncoated substrate.